1. Field of the Invention
The present invention relates to a method for wireless data transmission between a base station and one or more transponders.
2. Description of the Background Art
Transmission methods between one or more base stations, or readers, and one or more transponders are used in, for example, contactless identification systems or radio frequency identification (RFID) systems. Sensors, for example for temperature measurement, may also be integrated in the transponders. Such transponders are typically also referred to as remote sensors.
Transponders and their transmitting and receiving devices customarily do not have an active transmitter for data transmission to the base station. Such non-active systems are called passive systems if they do not have their own energy supply, and semi-passive systems if they have their own energy supply. Passive transponders take the energy they require for their supply from the electromagnetic field emitted by the base station.
In general, backscatter coupling is used to transmit data from a transponder to the base station using UHF or microwaves in the far field of the base station. To this end, the base station emits electromagnetic carrier waves, which the transmitting and receiving device in the transponder modulates and reflects appropriately for the data to be transmitted to the base station using a modulation method. The typical modulation methods for this purpose are amplitude modulation, phase modulation and amplitude shift keying (ASK) subcarrier modulation, in which the frequency or the phase position of the subcarrier is changed.
In German patent application 102 04 347, and in DE 101 38 217 A1 (which corresponds to U.S. Publication No. 20030133435, and which is incorporated herein by reference), methods for wireless data transmission between a base station and a transponder are described in which data packets to be transmitted comprise a header section, a data section with useful data to be transmitted, and a trailer section. The header section serves to set one or more transmission parameters, in particular, symbols that are found in the header section of the data packet are used for encoding and transmitting the useful data requiring transmission. In this context, a symbol is used to define or interpret the value of a character.
Such a symbol is customarily represented with the aid of a time interval between two sequential symbol delimiters or field gaps, or so-called “notches,” in the header section. With amplitude modulation, such a symbol delimiter can be created, for example, by suppressing or blanking the carrier signal. Another option for amplitude modulation is what is called double-sideband modulation having a suppressed carrier, in which the frequency of the carrier signal in the resulting spectrum is suppressed. The transponder decodes received data packets on the basis of the symbols contained in the header section, or using the time periods associated with the symbols, in that it determines the value of a character by comparing its time period with the time periods of the symbols.
The symbol delimiters are typically detected in the transponder using what are known as receiver signal strength indicator (RSSI) circuits. RSSI circuits recognize an attenuation of the field emitted by the base station and then generate a suitable signal for analysis in the transponder. As a result of interference sources in the environment of RFID systems, for example conventional GSM-based mobile radio systems, it is possible for the transponder to incorrectly detect symbol delimiters that were not generated by the base station. Another cause for the occurrence of such interference can be a frequency jump in the electromagnetic field emitted by the base station when a frequency hopping mechanism is used. Motion of the transponder in the environment of the base station can also cause the detection of a symbol delimiter as a result of the prevailing field strength distribution.
When such interference occurs during a data transmission or during a message between the base station and the transponder, the result is generally CRC errors or bit count errors, which communication participants detect in the security layers of the transmission protocol. The communication participants may then repeat the faulty message, for example.
When a symbol delimiter is incorrectly detected outside of a message, the transponder typically interprets this as the start of a message. The transponder then waits for subsequent symbol delimiters in order to process the message. Since no symbol delimiters are transmitted by the base station in this case, a blockage in a receiver unit of the transponder occurs. In order to avoid such blockages, monitoring is performed in the transponder during the data transmission, i.e. at least after the receipt of a first symbol delimiter, to determine whether a time period between two successive symbol delimiters transmitted by the base station exceeds a maximum time or whether no additional symbol delimiters are received within the maximum time. If the maximum time is exceeded, the receiver unit of the transponder is reset. In this way, a blockage or lock-up of the receiver unit resulting from an incorrectly received symbol delimiter is prevented.
The maximum time is typically chosen statically such that worst case conditions are satisfied. To this end, the maximum time is set to a long duration, with the result that a correspondingly long time elapses between the occurrence of interference and the resetting of the receiver unit. Thus, data throughput decreases.